In substrate processing in general and in PVD (sputtering) processing in particular, particulates which are present and are generated in the processing chamber can contaminate and destroy the substrate being processed. When such particulates (also known as "free" particulates) land on the substrate being processed, they contaminate a small area of the substrate which can be discarded when die cut into separate chips. However, when a large substrate is intended for subsequent use as a single item (e.g. as a flat panel display), one defect causes the whole unit to be rejected.
The contaminating particulates originate from several sources. Incomplete or defective cleaning of the chamber allows particulates to remain in the chamber and cause contamination. However, even when the processing chamber is clean, contaminants can be and are generated during the sputtering process. One type of contaminating particulate originates from sputter deposited material which has deposited itself on processing chamber surfaces other than the substrate intended for deposition, and subsequentially detaches (peels off or falls off)from the location inside the vacuum processing chamber where it originally had been deposited. These particulates are usually cool, multi-molecular sized specks of sputter deposited material which were hot during the sputtering process, but have since cooled as a result of their contact with surrounding surfaces. But unlike the hot material being sputter deposited in which molecular sized particles of the material (at the high temperatures 250.degree. to 400.degree. C.) adhere freely to the already hot substrate being sputtered, When cool specks (particulates) land on and are embedded in the substrate, such specks can create defects which cause rejection of the substrate.
Another source of particulates is electrical arcing between the highly charged (biased) target and its surrounding uncharged (grounded) pieces. Arcing occurs in PVD processing chambers at locations between the edge of the target and surrounding surfaces (usually a shield enclosing the target and protruding into the space adjacent to the target which is known as "the dark space ring or groove"). Arcing between adjacent pieces causes a severe localized temperature spike which in most cases releases molecules of one or both of the materials between which the spark arcs. If the released molecules settle on the substrate, at best, they create a slight but acceptable anomaly in the coating pattern, or at worst when a particulate is a foreign material then the substrate will be contaminated and will have to be rejected.
In a PVD processing chamber, the target containing the material to be sputtered is generally flat and located parallel to the substrate which is to be sputter deposited. Sputtering involves the ionization of gas (e.g. argon) molecules in the processing chamber. The gas molecules are electrically ionized as a result of an electrical bias, usually a DC bias. Once ionized the positive ions bombard the negative biased target causing the target material to be released into the chamber as molecular size ballistic particles. In the rarified vacuum atmosphere of the vacuum chamber the target molecules travel nearly unobstructed until they reach the substrate being sputter deposited, which is located just a short distance away.
This sputtering activity coats the substrate as desired by the process, but since the target material being sputtered is emitted from the target in all directions, the surfaces in the processing chamber around the substrate also tend to become coated with sputter deposited material (e.g. the shadow frame, the chamber shield). Because these surrounding surfaces are initially generally cold, i.e. at ambient or room temperature, in contrast to the temperature of the sputter deposited material which is at 250.degree. to 400.degree. C., upon contact the sputter deposited material rapidly cools to the lower temperature of the process chamber surfaces surrounding the substrate. The initial contact and adhesion between the high temperature sputter deposited material and the cool chamber surfaces surrounding the substrate creates a contact area between the two materials. As the sputter deposited material cools it tends to contract on the cool internal surface of the chamber. Contraction of the new sputter deposited material is restrained by the adhesion between the two materials at the initial contact area. When the tension in the sputter deposited material increases (as a result of larger and larger areas of the inside the processing chamber having been coated) some of the sputter deposited material eventually peels off the chamber surface. Each instance when sputter deposited material peels off the chamber surface creates another particulate which can contribute to contamination. In acknowledgment of this problem PVD chambers are constructed with "shield" pieces which act as a lining for the processing chamber. A shadow frame and shield (collectively "shield")line the inside of the processing chamber substantially between the edge of the target being sputtered and the edge of the substrate intended for deposition. The sputter deposited material then coats the inside of the "shield" and not the inside of the chamber wall. The "shield" can then be easily removed and cleaned or replaced, which reduces the wear and tear on the chamber wall which would be encountered if the walls of the processing chamber were to be continuously exposed to the ionized process gas and required a thorough cleaning after a predetermined number of processing cycles.
However, even in those instances where a "shield" is used, the peeling phenomenon described above eventually occurs when sputtering material on the "shield" surface builds up to and beyond the capacity of the "shield" to retain it without peeling off. Bead blasting is a technique commonly used to improve the adhesion between the sputter deposited material and the "shield" surface. Bead blasting provides additional surface area for the sputter deposited material and can effectively provide a mechanical coupling between the sputter deposited material and the surface of the "shield" so that the sputter deposited material is retained on the surface of the "shield" and does not peel off creating particulates in the processing chamber.
Arcing around the edge of the sputtering target can also create particulates. Arcing is induced when the bias voltage between the target and a nearby grounded (or dissimilarly biased) member is greater than a known function of a multiplicative product of the gas pressure and gap spacing between the target and that nearby grounded (or dissimilarly bias) member. The known numerical relationship is given by Paschen's curves (details of which are considered below in discussing FIG. 16). The curve shows conditions which are conducive to arcing between the target material and the surrounding shield in the "dark space ring" for a particular gas. An arc jumps between the edges of the biased target and grounded pieces such as the "shield." The arc causes specks to erupt from the material. Such specks can and do contaminate the substrate.
The expansion and contraction of process chamber structures due to changes in their temperature affects the gap or clearance between pieces across which arcing might occur. More specifically, because of the differences in temperature among the target, the process chamber wall, and the shield, it is possible for an arc to occur not only at an inside edge of the dark space groove between the edge of the target and the edge of the shield facing the target material, but also at an outside edge of the dark space groove surrounding the target, such that when the target shrinks or the shield expands the magnitude of the gaps created between adjacent pieces cause arcing.
A common solution to this arcing problem is to keep the clearance between adjacent pieces (i.e. the shield and the target) under the low end of Paschen's curve to prevent arcing. However, it is difficult to keep a constant clearance between the shield and the target material since the shield expands and its temperature rises due to exposure to ionized gas particles and sputtered material during the process. It is especially difficult to maintain a desired range of clearance dimensions when sputtering is being done for liquid crystal display applications. In these applications, the size of the area being sputtered is large (470 mm.times.370 mm), requiring a long and wide shield (outside dimension 660 mm.times.570 mm) around the perimeter of target being sputtered. The larger dimensions create larger movements due to differential thermal expansion which are hard to design around. Further, even when designs are created which in theory provide acceptable performance at all temperatures, a slight misalignment or offset of the shield from the target material during assembly of the processing chamber can and does create a clearance at one side of the chamber which is conducive to arcing, and thus creates particulates. The thermal cycling of "shield" elements from energy supplied and lost as sputtering is turning on and off, puts the adhesive bond between the sputter deposited material and the "shield" pieces to a test. Weakly bonded specks will soon fall or peel off as a result of thermal cycling, exacerbating the problem of particulates in the processing chamber.
Particulates created either by sputter deposited material peeling off from process chamber surfaces or arcing are unacceptable as particulate contamination affects the yield rate of semiconductor production. These difficulties need to be overcome in order to increase the yield in production of sputtered substrates and reduce or eliminate substrate rejection because of particulate contamination where the particulates originate from the sputtering process and not from any imperfect prior cleaning process.